staging: r8712u: Replace '--help--' with 'help'

This patch replace '--help--' in Kconfig to silence checkpatch.pl
warning:
WARNING: prefer 'help' over '---help---' for new help texts

Signed-off-by: Wentao Cai <etsai042@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>